when creating the websocket servers to enable deflate compression on
websocket frames. This is relevant to cryptonomex/graphene#540 because
the spammed data is higly compressible. In my tests, it reduces bandwidth
for a single idle node by a factor of ~16, from 577kbps down to 36kbps.
This doesn't require any changes to the wallets, simply upgrading the
public servers will begin sending compressed data to all clients that
support it.
Note: this commit adds a dependency on zlib for non-apple platforms
(it was already required on apple)
